The present invention relates generally to data backup in a distributed storage system comprising two different volumes located in different systems, and more particularly to comparing data between the two volumes.
In storage systems, it is common to have two or more volumes containing the same data. In general, one copy is the production volume (the active volume) which is the copy where data are written to and read from, while at least one secondary volume is a copy of the production volume.
In some storage systems (active-active systems), both copies of the data may act as production volumes. Either system may be treated as a primary system, while the other system serves as a secondary system (for example, to back each other up, or for load sharing). I/O activity (particularly, write activity) at either system is duplicated at the other system to keep the volumes synchronized.
A secondary volume can be used as back-up for disaster recovery, for archival purposes, for a point-in-time “snapshot”, for testing while ensuring the production copy is not corrupted, etc. In at least the case where a back-up copy for disaster recovery is being maintained, both copies (primary and secondary) have to remain continuously synchronized.